1. Field of the Invention
The present invention relates to a bracket and an orthodontic appliance for use in an orthodontic treatment.
2. Description of the Related Art
An irregular row of teeth detracts from ones appearance. In addition, plaque is likely to accumulate on teeth in an irregular row, causing dental caries and pyorrhoea alveolaris. In order to prevent such problems, orthodontic treatments have been widely conducted.
Commonly, an orthodontic treatment is conducted by the following steps. First, a bracket is attached onto the surface of tooth to be treated by an adhesive and the like. The bracket is formed with a groove to be engaged with an arch wire. In this state, the arch wire applies a pushing force, a pulling force or a twisting force onto the tooth via the bracket (hereinafter, such forces are referred to as an orthodontic force). Due to the application of the orthodontic force, the tooth is moved to a desired position in a desired direction. The bracket may also have a hook. When two or more teeth are respectively provided with a bracket with a hook, the brackets are connected to each other by putting a rubber ring or a spring around the hooks, and at the same time an arch wire is engaged to the grooves of the brackets. In this state, an orthodontic force is applied to the tooth to be treated by the arch wire and the rubber ring, thereby moving the tooth to the desired position in a desired direction.
Conventionally, a bracket is mainly made of metal (Conventional Example 1). The metal bracket is shiny and is conspicuous, and therefore, the orthodontic treatment using such brackets may be considered unaesthetic. To avoid such a problem, in recent years a bracket made of a synthetic resin or ceramics has been suggested (Conventional Example 2). As the synthetic resin bracket and the ceramics bracket are a milk white color, transparent, or semi-transparent, the appearance of the bracket is inconspicuous when attached to a tooth.
However, the synthetic resin bracket and ceramics bracket have many physical disadvantages as compared with the metallic bracket. For example, when an arch wire is engaged to the groove of the bracket, large friction is generated between the arch wire and the groove of the housing. Due to the large friction, the arch wire cannot smoothly slide in the groove, and the inner surface of groove in contact with the arch wire may be worn out or cracked. As a result, an orthodontic force is not properly applied to the tooth to be treated.
To reinforce the portion of the groove brought into contact with the arch wire, a bracket including a housing made of synthetic resin or ceramics and a metal body accommodated thereto has been suggested (Conventional Example 3). The metal body is formed with a groove therein adapted for being engaged with an arch wire. As described above, the inner surface of the groove brought into contact with the arch wire is likely to be worn out. If the inner surface of the groove is worn out, a proper force application is impossible, so that the tooth to be treated cannot be moved in a desired direction. In contrast, in Conventional Example 3, the bracket includes a metal body in the housing. The metal body is excellent in abrasion resistance, and an arch wire can smoothly slide in the metal body. Accordingly, a proper orthodontic force can be continuously applied to the tooth for a long period of time.
FIG. 13 is a perspective view showing a bracket 52 without a hook according to Conventional Example 3. FIG. 14A is a perspective view showing a state in which an arch wire 2 is engaged to a groove of the bracket 52. FIG. 14B is a cross-sectional view showing the bracket 52, taken along the line XIV.sub.B --XIV.sub.B in FIG. 14A.
As described above, the bracket 52 includes a housing made of synthetic resin and formed with a groove therein, and a metal body formed with a groove and accommodated to the housing. In FIGS. 13, 14A, and 14B, the bracket of Conventional Example 3 is a twin-type bracket having two pairs of wings 51.sub.Ru, 51.sub.Rd and 51.sub.Lu, 51.sub.Ld.
In the orthodontic treatment, the housing 52 is attached to the tooth to be treated (not shown) by its adhesion side 52a, and the arch wire 2 is engaged to the groove 50. In this state, as shown in FIG. 14A, a fastening wire 56 such as metal wire is put around the pair of wings 51.sub.Ru, 51.sub.Rd to tightly fix the arch wire 2 to the bracket 52. Or alternatively, the fastening wire may be put around both the pairs of wings 51.sub.Ru, 51.sub.Rd and 51.sub.Lu, 51.sub.Ld. In addition, the fastening wire 56 is not limited to a metal wire, but may be other parts such as a rubber ring.
FIG. 15 is a perspective view showing a bracket 62 with a hook according to Conventional Example 3. The bracket 62 is a single-type bracket formed with a pair of wings 61, 61 as a one-piece unit. The bracket 64 includes a housing made of synthetic resin formed with a groove, and a metal body 65 accommodated thereto.
In an orthodontic treatment, a bracket 62 is attached to the surface of a tooth to be treated (not shown), and an arch wire is engaged to the groove of the metal body 65. In this state, a fastening wire is put around the wings 61, 61 so as to firmly fix the arch wire to the bracket 62. At the same time, the bracket 62 is connected to another bracket 62 attached to another tooth by putting a rubber ring around their hooks, so that these teeth may be moved toward each other.
There are various shapes of brackets. Examples thereof include: a twin-type bracket formed with two pairs of wings without hook, such as shown in FIGS. 13 and 14; a single-type bracket formed with a pair of wings without hook; a single-type bracket formed with a pair of wings, one of which has a hook on its end, such as shown in FIG. 15; and a twin-type bracket formed with two pairs of wings one of which has a hook on its end. These may be used in combination in an orthodontic treatment.
An orthodontic treatment is conducted by the following steps. In the first step, an irregular row of teeth is roughly corrected (hereinafter, this step is referred to as a rough treatment step). At this stage, the arch wire need not to be firmly fixed to the bracket, and there is some clearance between the arch wire and the groove of the bracket. Without the clearance, an excessive force is applied to the tooth. The excessive force to the tooth not only causes inconvenience to the patient, but also moves the tooth to an extent beyond that necessary. As a result, the local treatment is excessive, and the row of teeth cannot be adjusted in a desirable manner.
In order to avoid the above-described disadvantage, in a rough treatment step, an arch wire having a round cross-section is generally used, and the arch wire is loosely fixed to the bracket. More specifically, the arch wire has a round cross-section and the groove is in a form of U-shaped square enclosed with flat side surfaces and a flat bottom surface. With this structure, when engaged to the groove, the arch wire is brought into point contact with the groove. In addition, the arch wire having a round cross-section easily changes its contact point with the groove when being twisted and the like. Furthermore, in the rough treatment step, a fastening wire is loosely put around the wings of the bracket. Therefore the arch wire and the bracket are loosely fixed to each other. In this state, the row of teeth is roughly adjusted to a desired alignment.
When the rough adjustment of the row of teeth is completed, the row of teeth is precisely adjusted (hereinafter, this step is referred to as a precise treatment step). As shown in FIG. 14A, an arch wire 2 used in the precise treatment step has a square cross-section. The arch wire 2 is engaged to the groove 50 of the bracket 52, and in this state, the fastening wire 56 is strongly put around the wings 51.sub.Ru, 51.sub.Rd so as to tightly fix the arch wire 2 to the bracket 52. As the arch wire 2 having a square shape is strongly pushed toward the bottom of the groove 50, the arch wire 2 does not roll inside the groove. In this state, a torque is continuously applied to a tooth to be treated at a desired torque angle .theta., whereby the tooth is moved to a desired position in a desired direction.
In general, a single-type bracket is advantageous for its small size. However, the single-type bracket is attached to the tooth in such a manner that the center of its wing along a direction perpendicular to the groove is positioned to the center of the tooth along the direction perpendicular to the groove. Therefore, there is a disadvantage that the right and left portions of one tooth cannot be treated by an independent force. Contrary to this, a twin-type bracket has an advantage that, when an arch wire is fixed to the bracket, the two pairs of wings of the bracket are used individually or in combination, depending on the necessity. In this manner, the tooth can be easily controlled to move in various directions. The use of the twin-type bracket is especially desirable in a precise treatment step.
However, as compared with the twin-type bracket made of metal, the twin-type bracket including a synthetic resin housing and a metal body, such as that of Conventional Example 3, has disadvantages as follows.
FIG. 16 is a cross-sectional view showing a bracket 52 of FIG. 14A taken along the line XVI--XVI. The identical components to FIG. 14A are designated by the same reference numerals, and the description thereof will be omitted.
The bracket 52 includes a metal body 53 produced by bending a flat metal plate into a U-shape. Therefore, the metal body 53 is enclosed with flat surfaces. When an arch wire 2 is engaged to the groove 50 fitted with the metal body 53 and then a fastening wire 56 is put around the wings 51.sub.Ru, and 51.sub.Lu, the arch wire 2 is firmly fixed to bracket 52 only at the one end portion .beta.. Therefore, the fastening wire 56 cannot firmly fix the arch wire 2 to the bracket 52 at the middle portion of the metal body 53. For firm fixation, it is necessary to put the fastening wire 56 around both pairs of wings. When the fastening wire is put around both pairs of wings, the control of the row of teeth is difficult and insufficient.
If the firm fixation of the arch wire 2 to the bracket 52 is successfully conducted by putting the fastening wire 56 around only one pair of wings, a uniform force is applied to the bracket over the entire length of the groove of the metal body 53. That is, the force is equivalent to that of the case of putting a fastening wire around two pairs of wings. As a result, it is impossible to treat the right and left portions of the tooth by an independent force. Practically, the conventional twin-type bracket only has a function of the single-type bracket.
As an attempt to solve the above-described problem, it has been suggested that a metal body is provided to the groove of housing only at the portions corresponding to the wings, and the middle portions of the groove of the housing is not provided with a metal body. With this structure, however, the bracket has poor strength and therefore is easily bent.
When a bracket is entirely made of metal, it may be advantageously used as a twin-type bracket. The twin-type bracket made of metal can be used in many applications. In contrast, as has already described above, the bracket of Conventional Example 3 including a synthetic resin housing and a metal body accommodated thereto has several problems.
In addition, the brackets 52, 62 of Conventional Example 3 have a further problem. That is, by being enclosed with the flat surfaces, the metal bodies 53, 65 cannot resist a force applied to their side surfaces in the width direction. Therefore, the metal body may be easily deformed.
Moreover, the bracket of Conventional Example 3 having a hook has further problems as follows. As described above, the bracket made of synthetic resin or ceramics has a strength smaller than the bracket made of metal. Therefore, the bracket made of synthetic resin and ceramics is required to have a hook larger in size than that of the bracket made of metal (see FIG. 15). However, when such a bracket is attached to the tooth, a large hook is brought into contact with the gingiva, bucca, and lips, which gives a sense of discomfort to the patient. In addition, plaque tends to accumulate on the large hook, which makes the inside of the mouth insanitary.
The hook made of synthetic resin or ceramics cannot be bent. Therefore, even if the hook is brought into contact with the gingiva, it cannot be apart from the gingiva. This may cause inflammation of the gingiva.
Contrary to this, when the bracket with a hook is entirely made of metal (Conventional Example 1), the hook can be small in size while having a sufficient strength. In addition, the hook can be bent. Accordingly, in a practical treatment, a metal bracket with a hook is widely used. However, the appearance of the bracket as a whole is deteriorated when a metal bracket is used.